10 replies to this topic

[Leonard]

im gonna fit a centre main strap for the engine im building. apart from machining the bearing cap flat, is there anything else? i heard it needs to be line honed.

could i do the mainstrap with a lathe, and with a milling attachment, or does it need to be ground?

[chairchild]

Do it properly, with a mill

Edited by chairchild, 18 January 2008 - 10:34 AM.

[Leonard]

cheers mate. is the metal to hard for a milling tool then?

[Turbo Nick]

nope. i watched simon at morspeed do my mates cap for him and he used a lathe (pretty sure it was anyway, not too hot on big cutting machines )

I wouldn't risk using a grinder, way too primitive for a mains cap IMO.

[Dan]

I have to disagree with that.

The centre strap needs to be very flat and absolutely not tapered in order for the bearing cap to be properly clamped down. I wouldn't cut one with an angle grinder myself. And yes you will need to flatten off the cap and get the thing line bored afterwards, or at least inspected to check whether it's needed. The act of flattening the main cap weakens it, and then you strap a peice of steel across it. This can very easily close up the bearing housing a little. Remember the tollerance in this bearing is half a thou and the running clearance is only 1 thou (min) meaning it would be easy to go below that which would not be good for the life of the bearings.

But if others have done this successfully without taking the precaution then I'll happily accept their opinion.

[Sprocket]

Do it by hand with either an angle grinder, or abench-mounted grinder if you're feeling posh

What the F

Death to your engine if you do it that way

[Sprocket]

If you'r gonna have a costly line bore, then in my opinion you might as well spend £85 more and fit a four bolt cap for that added safety margin of a bullit proof bottom end. All depends on the power output and RPM potential.
£80 for the cap and bolts

£20 for the drilling and tapping of extra bolts

£120 for the line bore

[Oldskoolbaby]

Do it by hand with either an angle grinder, or abench-mounted grinder if you're feeling posh

Thats like boring your block out with a Dewalt hand drill, and honing it with wire wool. Theres only one way to build a decent motor and thats to make everything perfect and not cut corners. No point driving it a mile down the road for the bottom end to seize :nugget:

[chairchild]

I have to disagree with that.

The centre strap needs to be very flat and absolutely not tapered in order for the bearing cap to be properly clamped down. I wouldn't cut one with an angle grinder myself. And yes you will need to flatten off the cap and get the thing line bored afterwards, or at least inspected to check whether it's needed. The act of flattening the main cap weakens it, and then you strap a peice of steel across it. This can very easily close up the bearing housing a little. Remember the tollerance in this bearing is half a thou and the running clearance is only 1 thou (min) meaning it would be easy to go below that which would not be good for the life of the bearings.

But if others have done this successfully without taking the precaution then I'll happily accept their opinion.

Noted!!!


Never seen results of this being done - only heard of it as an option, so the input is appreciated

[Wil_h]

Are you doing this on a 1275 or a 998?

It's a bit trickier on a 998, thats all.

I found this out after letting an 'expert' build my engine, including fitting a centre main strap. I didn't realise he'd done it wrong till the engine lunched the thrust bearings, taking the block with them; the crank narrowly survived.

Wil

[Guess-Works.com]

I have a local engineering firm do my main caps, goes on a flatbed mill and couple of passes and it's done... only a fiver a pop, then buy the mainstrap from min-its...

See my other thread about this subject :saywhat: Clicky